The present invention relates to a partially pressure-sensitive recording paper produced by applying an ink comprising a uniform dispersion of microcapsules containing a solution of an electron-donating, colorless dyestuff, a thermally melting solid substance and a high-boiling organic solvent partially on a sheet of paper.
While the so-called business form such as slips for business, computor recording paper, etc. have come to be more and more complicated and diversified in recent years with the increase of business efficiency and the automation of business, of such slips and recording paper, there are many cases where they take the form of a copying paper including a plurality of sheets of paper.
In such cases, the pressure-sensitive recording paper is mostly used and generally is prepared by laminating an upper sheet (CB paper), on the lower side of which microcapsules containing a solution of a so-called leuco-type dyestuff as the core substance have been applied, with a lower sheet (CF paper) on the upper side of which acid clay or a resin presenting acidity (color-developer) has been applied. The thus prepared pressure-sensitive recording paper is to develop color by rupturing the microcapsules on the parts where a writing pressure or a typewriting pressure is applied, thereby making the dyestuff contact with the color-developer. On the other hand, in the pressure-sensitive recording paper which can produce a plurality of copied sheets at a time, one or several sheets of paper (CFB paper), on the upper side of which a color-developer has been applied and on the lower side of which microcapsules containing a dyestuff have been applied, are inserted between CB paper and CF paper and is used.
However, in the above case, since the microcapsules have been applied on the whole surface of the CB paper, for instance, in the case wherein specified numbers of the whole sheets are necessary to be copied, or in the case wherein only the specified part of a sheet is necessary to be copied, it is necessary to de-sensitize the color-developer except on the specified sheet or part by using a de-sensitizing ink purposely, to avoid the not-necessary copying. Accordingly, the above-mentioned technique would double the trouble, and it is inevitable that the microcapsules in the de-sensitized part become wasted.
In view of the above-mentioned troubles, it is considered that if it is possible to make only the necessary part retain the microcapsules, the partially pressure-sensitive recording paper can be obtained without using the de-sensitizing ink or without applying the expensive microcapsules on the unnecessary parts. Accordingly, a presentation of the partially pressure-sensitive recording paper prepared by applying the microcapsules only on the necessary part(s) with a method such as spot printing has been required.
In order to answer the requirement, methods for producing the partially pressure-sensitive recording paper by partially applying microcapsules have been developed.
At present, although the pressure-sensitive recording paper is usually produced by applying a slurry comprising microcapsules, a water-soluble binder and an additive on a paper using water as a dispersing medium, in the case where the partially pressure-sensitive recording paper is produced by applying this method with, for instance, spot printing, it is extremely difficult to obtain the product high in practicality, because of the partial creases formed at the time of drying the recording paper.
In order to avoid the above-mentioned defect, for instance, there is a method wherein the microcapsules are dispersed in an organic solvent containing a vehicle and the thus formed dispersion is partially applied on a sheet of paper by a printer of gravure or flexo-system, however, such a printer could not always be easily installed technically, areawise or moneywise. In addition, since such a method uses a large amount of an organic solvent, the method has defects such as polluting the working environment.
Further, there is another method for producing a partially pressure-sensitive recording paper, wherein after partially applying a photo-setting adhesive on a board, microcapsules are adhered to the part before the adhesive solidifies, and thereafter the adhesive is solidified. However, in this method, the adhesive is very expensive at present and there is a defect that the operation in the production thereof is complicated.
Still more, as conventional partially pressure-sensitive recording papers produced by using an ink of thermally melting type, those produced by partial printing using a so-called carbon ink, which is prepared by suspending a colored pigment such as carbon black in an ink of thermally melting type, have been well known. Although the pressure-sensitive recording paper produced by using such a carbon ink has been broadly used due to the simplicity of the printing procedure and the low cost, since the colored pigment has been mixed with a wax and the mixture has been simply applied, such a recording paper has a fatal defect of soiling hands and clothes of persons handling it.
As a method for producing a pressure-sensitive recording paper, wherein the above defects have been improved, a method of using microcapsules retaining a solution of a colorless dyestuff and a thermally melting substance has been proposed in Japanese Patent Publications No. 48-12255 (1973) and 57-53196 (1982).
The Patent Publications disclose a method for producing a pressure-sentitive recording paper by applying an ink comprising a mixture of microcapsules containing a solution of a dyestuff and a thermally melting substance on a surface of a sheet of paper. The largest defect of the pressure-sensitive recording paper obtained by the above method lies in the point that the color-developing property is extremely damaged and when such a pressure-sensitive recording paper is compared to the recording paper of which the whole surface is pressure-sensitive and which is obtained by the most common method for producing the pressure-sensitive recording paper, namely by applying an aqueous dispersion of microcapsules, the method shown in the Japanese Patent Publications cannot be said to be the method which can offer the fully practical pressure-sensitive recording paper in the points of the density of the developed color and the initial speed of color-developing.
Further, in Japanese Patent Application Laid-Open (KOKAI) No. 60-149489 (1985), a method for producing a partially pressure-sensitive recording paper by applying an ink, made by mixing an organic solvent of a relatively low boiling point, a thermally melting substance and microcapsules containing a solution of a colorless dyestuff, on a surface of a paper sheet has been proposed. The density of the developed color and the initial speed of color developing of the thus prepared partially pressure-sensitive recording paper are not inferior to those of the pressure-sensitive recording paper prepared by coating aqueous suspension of microcapsules on whole surface, however, the method has not yet solved the problem of an environmental pollution by the evaporation of the organic solvent of a low boiling point at the time of producing the ink and/or the pressure-sensitive recording paper.
Still more, in Japanese Patent Application Laid-Open (KOKAI) No. 59-164186 (1984), as a preceding technique of the invention, it is disclosed that an oily substance is mixed with a wax as is the case of preparing a conventional carbon ink, and after dispersing the microcapsules containing a dyestuff into the thus formed mixture to prepare an ink, and when the thus prepared ink is applied on a sheet of paper to make an upper sheet and the upper sheet is combined with a lower sheet to develop a color, the density of the developed color on this recording paper is high without any part loss of a letter. However, the Patent Application also states that on the other hand, when the upper sheet is coated by the ink containing the oily substance of a sufficiently large amount for obtaining a high density of developed color and a wax and the upper sheet and the lower sheet are combined, the color is developed by simply rubbing the thus combined sheets lightly with fingers, and accordingly such product has no practical value.
Further, in Comparative Example 2 of the Japanese Patent Application Laid-Open (KOKAI) No. 59-164186, it describes that a pressure-sensitive recording paper using 20 parts by weight of SAS oil N-296 (made by Nippon Petrochemical Co., an oil of diarylalkane series), 60 parts by weight of rice wax and 20 parts by weight of microcapsules was tested and although the color-developing property was favorable, it had many stains.
Still more, this Japanese Patent Applicatioh Laid-Open (KOKAI) No. 59-164186 (1984) discloses a technique for producing a pressure-sensitive recording paper without the stains by microencapsulating all or a part of the oily substance and preparing the ink by dispersing thus prepared microcapsules and the other microcapsules containing a precursor of a dyestuff in a wax.
Moreover, this reference (KOKAI No. 59-164186) defines the oily substance as at least one natural or synthetic oily substance and illustrates cottonseed oil, kerosene, paraffin, naphthenic oil, alkylated biphenyl, alkylated terphenyl, alkylated naphthalene, triarylmethane, chlorinated paraffin, diarylalkane, styreneoligomer which is liquid at ordinary temperature, silicone oil, phthalate ester, phosphate ester, sulfonate ester, diaryl ether, aryl alkyl ether, higher alkylated benzene and caster oil as the examples of the thus defined oily substance. However, the method of this reference (KOKAI No. 59-164186) cannot be said to be a favorable method in the points that the method uses a large amount of the expensive microcapsules, of which process for production is complicated.
Accordingly, there has not been proposed any method for obtaining the truly practical partially pressure-sensitive recording paper by simple method.
As a result of the present inventors' earnest studies for obtaining a partially pressure-sensitive recording paper which is excellent in color-developing property, has no fear of environmental pollution, is free from stains to the extent sufficient in practical use and can be produced by simple process with a low cost, it has been found out that a partially pressure-sensitive recording paper which is as high in quality and as low in cost as the recording paper which has been coated with the aqueous suspension of microcapsules on the whole surface area, can be obtained by using an organic solvent having a boiling point and aromatic ring carbons respectively in a definite range together with microcapsules and a thermally melting solid substance and by prescribing the weight ratio of the organic solvent to the thermally melting solid substance and the weight ratio of the thermally melting solid substance to the microcapsules, and on the basis of these findings, the present invention has been attained.